Hello all,
I am having problems with my Akai AM-2200.
First off full DC on output.
Found out that TR4 and 5 were not putting out the required voltages.
Replaced those and most of the PS caps.
That fixed the TR4 and 5 voltages but not the DC on output.
In circuit the rest of the transistors show no unusual shortcircuits etc.
The fuse resistors all measure ok but I will replace those later on.
Disconnected the power transistors; still DC on output.
I have now decided to replace driver and power transistors.
Here's what i have come up with:
Original power transistors:
2SB616 100V, 5A, 60W, 22MHz hfe 40-200
2SD586 100V, 5A, 60W, 15MHz hfe 40-200
Replace with:
2SB688 150V, 8A, 80W, 10MHz hfe 55-160
2SD718 120V, 8A, 80W, 12MHz hfe 55-160
Original drivers:
2SB560 (E,F) 100V, 700mA, 0.9W, 100MHz, hfe 100-320 (E+F)
2SD438 (E,F) 100V, 700mA, 0.9W, 100MHz, hfe 100-320 (E+F)
Replace with:
2SA1837 230V 1A 20W 70MHz hfe 100-320
2SC4793 230V 1A 20W 100MHz hfe 100-320
Do these seem ok?
Choices were also dictated by what I can get locally for cheap.
Also I intend to replace the 0.47 ohm emitter resistors with 0.22 ones and adjust the bias accordingly.
Also possible and ok?
Thanks in advance for your input.
Cheers,
Johan
I am having problems with my Akai AM-2200.
First off full DC on output.
Found out that TR4 and 5 were not putting out the required voltages.
Replaced those and most of the PS caps.
That fixed the TR4 and 5 voltages but not the DC on output.
In circuit the rest of the transistors show no unusual shortcircuits etc.
The fuse resistors all measure ok but I will replace those later on.
Disconnected the power transistors; still DC on output.
I have now decided to replace driver and power transistors.
Here's what i have come up with:
Original power transistors:
2SB616 100V, 5A, 60W, 22MHz hfe 40-200
2SD586 100V, 5A, 60W, 15MHz hfe 40-200
Replace with:
2SB688 150V, 8A, 80W, 10MHz hfe 55-160
2SD718 120V, 8A, 80W, 12MHz hfe 55-160
Original drivers:
2SB560 (E,F) 100V, 700mA, 0.9W, 100MHz, hfe 100-320 (E+F)
2SD438 (E,F) 100V, 700mA, 0.9W, 100MHz, hfe 100-320 (E+F)
Replace with:
2SA1837 230V 1A 20W 70MHz hfe 100-320
2SC4793 230V 1A 20W 100MHz hfe 100-320
Do these seem ok?
Choices were also dictated by what I can get locally for cheap.
Also I intend to replace the 0.47 ohm emitter resistors with 0.22 ones and adjust the bias accordingly.
Also possible and ok?
Thanks in advance for your input.
Cheers,
Johan
Attachments
My akai 1135r has the same trannies...but one channel was burnt and can't find exact parts so i had replaced most of the parts that i already have..except the LTP and vas tranny....my output was B778/D998. driver was replaced with bd139/140 since my rails are only+/- 35v. also replaced the c945 vbe tranny with c3611 that was easily screwed in the heatsink but played a bit of the resistance in the vbe area to adjust the output bias normally to about 25ma..
Overall it was fine and no smoke....sounds good but can't compare to the previous trannies since it was brought to me not working.
As long as you comply to the ratings above you rails and dissipation...it will be okay but i guess you need some adjustment in the bias.
Overall it was fine and no smoke....sounds good but can't compare to the previous trannies since it was brought to me not working.
As long as you comply to the ratings above you rails and dissipation...it will be okay but i guess you need some adjustment in the bias.
To have full DC in the output will mean that one of your first stages transistors is shorted
another is that any of the transistors you replaced is oriented the wrong way
From experience i can tell you that even though the choice of drivers is very good these will not work in this amplifier since previous stages will not drive them properly and or compensation is designed fro the specific transistor and specs
Obtain original drivers its very possible
Notice that if you have oscillation coming from the choice drivers you are going to smell it before you hear it if you put load in the amplifier there you need a scope or original drivers
Regards
Sakis
another is that any of the transistors you replaced is oriented the wrong way
From experience i can tell you that even though the choice of drivers is very good these will not work in this amplifier since previous stages will not drive them properly and or compensation is designed fro the specific transistor and specs
Obtain original drivers its very possible
Notice that if you have oscillation coming from the choice drivers you are going to smell it before you hear it if you put load in the amplifier there you need a scope or original drivers
Regards
Sakis
Ok,
thanks all for the input.
I was kind off afraid of the answers Sakis provides and of course Jon is right.
My intention was to replace as much as possible since most parts (that I chose) are not expensive, but tomorrow I will check first stage transistors first and report back.
If faulty would you think BC550C would be ok?
Thanks again,
Cheers,
Johan
thanks all for the input.
I was kind off afraid of the answers Sakis provides and of course Jon is right.
My intention was to replace as much as possible since most parts (that I chose) are not expensive, but tomorrow I will check first stage transistors first and report back.
If faulty would you think BC550C would be ok?
Thanks again,
Cheers,
Johan
ksc1845 would be a useable replacement for the ltp transistors and should have the same pinout. Remember that both of them must be replaced together.
Hi Johan,
The input pair should be matched for beta for starters. You may not have a short to cause this. You also need to check the DC level shifter (TR3?) and have a look at the negative sides resistors that are supposed to work like a current source. An open here might run the output to one rail. You didn't say if the offset was positive or negative.
-Chris
The input pair should be matched for beta for starters. You may not have a short to cause this. You also need to check the DC level shifter (TR3?) and have a look at the negative sides resistors that are supposed to work like a current source. An open here might run the output to one rail. You didn't say if the offset was positive or negative.
-Chris
Progress but not as I'd expected.
Started to measure voltages near the first 3 transistors: all very near positive rail, so not good.
Figured it was missing ground connection.
It's supposed to get this through volume, tone-control and input boards but it wasn't.
Then I found that the screw that secures the groundconnection on the input board was not fully tightened. Replaced this with a new better fitting one and voila Job Done.
Feel a bit stupid but I guess it's all part of the learning curve.
Next is a NAD 3220PE which does have blown output and driver transistors.
Thanks again for all your input,
Cheers,
Johan
Started to measure voltages near the first 3 transistors: all very near positive rail, so not good.
Figured it was missing ground connection.
It's supposed to get this through volume, tone-control and input boards but it wasn't.
Then I found that the screw that secures the groundconnection on the input board was not fully tightened. Replaced this with a new better fitting one and voila Job Done.
Feel a bit stupid but I guess it's all part of the learning curve.
Next is a NAD 3220PE which does have blown output and driver transistors.
Thanks again for all your input,
Cheers,
Johan
Attachments
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Hi Johan,
Don't feel stupid about that! You found a really tough problem. After being burned a couple times myself, I generally start troubleshooting by tightening all screws after I have confirmed the fault. Now you will too! 🙂
NAD, no comment. I have repaired many, but it is a very low quality thing that should never have been allowed to market in my view.
At least this one is pretty much your standard amplifier, quite a step up from the normal stuff they put out. Expect a very positive temperature coefficient for bias current. Restricting airflow to the unit is enough to coax it into thermal runaway during a party. I guess there is one really good thing about this unit. It absolutely does need every trick in the book to improve performance. Practically anything you do down that road will improve the sound quality. The question is, what will you use it for? To keep and listen to? Pull out all the stops and install current sources, match transistors and replace those nasty ceramic capacitors. Try to fit modern output transistors while you are at it. If it will be used for garage sound ... I would probably fix it right up, but just running as it once did might be enough. Depends on you and the speakers. In stock form it is not a nice sounding amplifier though.
-Chris
Don't feel stupid about that! You found a really tough problem. After being burned a couple times myself, I generally start troubleshooting by tightening all screws after I have confirmed the fault. Now you will too! 🙂
NAD, no comment. I have repaired many, but it is a very low quality thing that should never have been allowed to market in my view.
At least this one is pretty much your standard amplifier, quite a step up from the normal stuff they put out. Expect a very positive temperature coefficient for bias current. Restricting airflow to the unit is enough to coax it into thermal runaway during a party. I guess there is one really good thing about this unit. It absolutely does need every trick in the book to improve performance. Practically anything you do down that road will improve the sound quality. The question is, what will you use it for? To keep and listen to? Pull out all the stops and install current sources, match transistors and replace those nasty ceramic capacitors. Try to fit modern output transistors while you are at it. If it will be used for garage sound ... I would probably fix it right up, but just running as it once did might be enough. Depends on you and the speakers. In stock form it is not a nice sounding amplifier though.
-Chris
Hello Chris,
thanks for the kind words.
And yes lesson learned; but baffled that it relies on the chassis to connect the ground of the boards.
The NAD is probably not a keeper, depending on how good the one I have is.
Bought it defective. I fixed the broken channel, but now the other channel needs tlc; both dc offset and bias are not stable. I put it aside for now and am now working on an Electrocompaniet AW75 Anniversary Edition.
Progress is steady but when I hit a wall I'll report back (in separate thread).
Having fun learning and the feeling when succesful can be addictive...
Cheers,
Johan
thanks for the kind words.
And yes lesson learned; but baffled that it relies on the chassis to connect the ground of the boards.
The NAD is probably not a keeper, depending on how good the one I have is.
Bought it defective. I fixed the broken channel, but now the other channel needs tlc; both dc offset and bias are not stable. I put it aside for now and am now working on an Electrocompaniet AW75 Anniversary Edition.
Progress is steady but when I hit a wall I'll report back (in separate thread).
Having fun learning and the feeling when succesful can be addictive...
Cheers,
Johan
Out of curiosity
Did you scope anything ? did you measure bias ? or is it ok if music is coming out of it ? did you notice if any of the changes you made produce any inaudible oscillation ? any offset ?
Kind regards
Sakis
Did you scope anything ? did you measure bias ? or is it ok if music is coming out of it ? did you notice if any of the changes you made produce any inaudible oscillation ? any offset ?
Kind regards
Sakis
Hi Johan,
You have been warned.
Sakis! A warm greeting for another tortured soul who sees components when he closes his eyes! How are you these days old friend?
Good questions that only Johan can answer. Johan?
-Chris
Why ever do you think I know as much as I do now??? 🙂 There has to be some reason we enjoy taking on someone else s problems. Otherwise there wouldn't be such a thing as a "technician".
You have been warned.
Sakis! A warm greeting for another tortured soul who sees components when he closes his eyes! How are you these days old friend?
Good questions that only Johan can answer. Johan?
-Chris
To which amp are you referring?
The Akai is now working fine: before I figured out the loose screw I replaced a couple of transistors in the PSU and a few elco's and after readjusted bias.
The NAD now has one working channel and one with drifting bias and DC; put it away for now an get back to it in a couple of weeks.
The Electrocompaniet has 3 blown transistors in one channel that I'm in the progress of replacing (replacing all 4 pairs in that channel. Drivers measure ok).
No scoping done yet; I know the Electrocompaniet is prone to oscillating so will do when finished.
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If the AKAI amp is managed with the original outputs and drivers then you are done If you replaced them with the ones suggested in the first post you have troubles that you haven't found yet regardless what John says ...
@Chris
Many of us kept our thoughts next to you in the time you have been away ... I wish you well and have a grate time .All the best for the family too !
We are doing very fine in between crises business has never been better , we had a target of 500 audio devices to be repaired from our second shop 2 years ago this target was covered by 750 aprx devices for the first year and now it has been stabilized to a bit over 1000 devices per year ..
So yes !!! we are doing very well
Hours are pressing coming in the office 9.30-10.00 and often have to go something like 1.00 in the night
Kids dog and wife are doing very well ... so that's about it !!
Keep Going Chris !!
warm regards
Sakis
@Chris
Many of us kept our thoughts next to you in the time you have been away ... I wish you well and have a grate time .All the best for the family too !
We are doing very fine in between crises business has never been better , we had a target of 500 audio devices to be repaired from our second shop 2 years ago this target was covered by 750 aprx devices for the first year and now it has been stabilized to a bit over 1000 devices per year ..
So yes !!! we are doing very well
Hours are pressing coming in the office 9.30-10.00 and often have to go something like 1.00 in the night
Kids dog and wife are doing very well ... so that's about it !!
Keep Going Chris !!
warm regards
Sakis
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